REPRODUCTION OF TISSUES. 3 



ply and develop. New blood capillaries, however, sprout 

 out from the sides of old, and new epidermis seems only 

 to be formed by the multiplication of epidermic cells; hence 

 the practice, recently adopted by surgeons, of transplanting 

 little bits of skin to points on the surface of an extensive 

 burn or ulcer. In both blood capillaries and epidermis the 

 departure from the primary undifferentiated cell is but 

 slight; and, as regards the cuticle, one of the permanent 

 physiological characters of the cells of the rete mucosum is 

 their multiplication throughout the whole of life; that is 

 a main physiological characteristic of the tissue: the same 

 is very probably true of the protoplasmic cells forming the 

 walls of the capillaries. .Nerve-fibres are highly differenti- 

 ated, yet nerves arc rapidly regenerated after division. 

 The branch of the nerve-cell (axis cylinder) grows again; 

 and amoeboid wandering cells make a new medullary 

 sheath. In Mammals, muscular and glandular tissues 

 seem never to be reproduced. 



We find, then, as we ascend in the animal scale a dimin- 

 ishing reproductive power in the tissues generally: with the 

 increasing division of physiological labor, with the changes 

 that fit pre-eminently for one work, there is a loss of other 

 faculties, and this one among them. The more specialized 

 a tissue the less the reproductive power of its elements, and 

 the most differentiated tissues are either not reproduced at all 

 after injury, or only by the specialization of amoeboid cells, 

 and not by a progenitive activity of survivors of the same 

 kind as those destroyed. In none of the higher animals, 

 therefore, do we find multiplication by simple division, or 

 by budding: no one cell, and no group of cells used for the 

 physiological maintenance of the individual, can build up 

 a new complete living being; but the continuance of the 

 race is specially provided for by setting apart certain 

 cells which shall have this one property cells whose duty 

 is to the species and not to any one representative of it an 

 essentially altruistic element in the otherwise egoistic whole. 



Sexual Reproduction. Income cases, especially among 

 insects, the specialized reproductive cells can develop, each 

 for itself, under suitable conditions, and give rise to new 



